Traditionally, digitally controlled color printing capability is accomplished by one of two technologies. Liquid, such as ink, is fed through channels formed in a print head. Each channel includes a nozzle from which droplets are selectively extruded and deposited upon a medium.
The first technology, commonly referred to as “droplet on demand” printing, provides droplets for impact upon a recording surface. Selective activation of an actuator causes the formation and ejection of a flying droplet that strikes the print media. The formation of printed images is achieved by controlling the individual formation of droplets. For example, in a bubble jet printer, liquid in a channel of a print head is heated creating a bubble that increases internal pressure to eject a droplet out of a nozzle of the print head. Piezoelectric actuators, such as that disclosed in U.S. Pat. No. 5,224,843, issued to VanLintel, on Jul. 6, 1993, have a piezoelectric crystal in a fluid channel that flexes when an electric current flows through it forcing a droplet out of a nozzle.
The second technology commonly referred to as “continuous stream” or “continuous” printing, uses a pressurized liquid source which produces a continuous stream of droplets. Conventional continuous printers utilize electrostatic charging devices that are placed close to the point where a filament of working fluid breaks into individual droplets. The droplets are electrically charged and then directed to an appropriate location by deflection electrodes having a large potential difference. When no print is desired, the droplets are deflected into a liquid capturing mechanism commonly referred to as a catcher, an interceptor, a gutter, etc. and either recycled or disposed of. When print is desired, the droplets are not deflected and allowed to strike a print media. Alternatively, deflected droplets may be allowed to strike the print media, while non-deflected droplets are collected in the capturing mechanism.
As conventional continuous printers utilize electrostatic charging devices and deflector plates, they require many components and large spatial volumes in which to operate. This results in continuous print heads and printers that are complicated, have high energy requirements, are difficult to manufacture, and are difficult to control.
U.S. Pat. No. 3,709,432, issued to Robertson, on Jan. 9, 1973, discloses a method and apparatus for stimulating a filament of working fluid causing the working fluid to break up into uniformly spaced droplets through the use of transducers. The lengths of the filaments before they break up into droplets are regulated by controlling the stimulation energy supplied to the transducers, with high amplitude stimulation resulting in short filaments and low amplitudes resulting in long filaments. A flow of air is generated across the paths of the fluid at a point intermediate to the ends of the long and short filaments. The air flow affects the trajectories of the filaments before they break up into droplets more than it affects the trajectories of the droplets themselves. By controlling the lengths of the filaments, the trajectories of the droplets can be controlled, or switched from one path to another. As such, some droplets may be directed into a catcher while allowing other droplets to be applied to a receiving member.
Commonly assigned U.S. Pat. No. 6,554,410 issued in the name David L. Jeanmaire et al. on Apr. 29, 2003, discloses so-called “stream” continuous-jet printing wherein nozzle heaters are selectively actuated at a plurality of frequencies to create the stream of droplets having the plurality of volumes. A force is applied to the droplets at an angle to the stream to separate the droplets into printing and non-printing paths according to drop volume. The force is applied by a flow of gas. This process consumes little power, and is suitable for printing with a wide range of inks.
Stream printing can be implemented in either of two complementary modes. The first is the so-called “small-drop” mode in which small droplets are directed to the image receiver and larger drops are captured by a gutter. In the second, “large-drop” mode, small droplets are guttered, while larger drops impact upon the image receiver. While high throughput and small drop size are desired characteristics of a printing system, these characteristics tend to be mutually exclusive in prior art “small-drop” or “large-drop” printers. Small-drop mode printers print with the smallest possible drop size, but cannot normally reach 100% of liquid utilization. Typically, a system running in small-drop mode has a liquid utilization factor less than 50%. On the other hand, in large-drop mode, liquid utilization can reach 100% at the expense of a larger size printing droplets, at least twice the size of the small-drop mode printers.